Water Confinement on Polymer Coatings Dictates Proton-Electron Transfer on Metal-Catalyzed Hydrogenation of Nitrite.

Abstract

Enzymes can precisely control the speed and selectivity of chemical reactions by modifying locally the solvent-reactant interactions. To extrapolate these attributes to heterogeneous catalysts, we have employed thermoresponsive poly n-isopropylacrylamide (p-NIPAM) brushes bonded to silica spheres containing palladium. These polymers can form hydrogen bonds with water molecules at low temperatures (<32 °C) allowing the polymer to stay swollen. Detailed reaction kinetics of nitrite hydrogenation showed that p-NIPAM decreases the apparent activation barrier by a factor of 3 at low temperatures. Diffusion-ordered spectroscopy nuclear magnetic resonance and ab initio molecular dynamics simulations showed that when p-NIPAM is present, water molecules near the surface are less mobile. This confinement perturbs the water interaction with the metal, reducing the barrier for the proton-electron transfer reduction of nitrite. Notably, this enhancement vanishes at high temperature as the polymer collapses on itself exposing the Pd to unconfined water. The fully reversible nature of this process opens the door for creating homeostatic catalysts with controlled water-confinement.